Dyskeratosis congenita (DC) is a bone-marrow failure syndrome characterized by early onset of reticulate skin hyperpigmentation, nail dystrophy, hyperkeratotic palms, hypotrichosis, atresia of the lacrimal ducts, poor dentition, and oral leukoplakia. The progressive bone-marrow failure affects the majority of cases. There is heterogeneity in the severity of the bone-marrow failure as well as in the range of associated anomalies, and an increased risk for a variety of malignancies. Some patients may be initially characterized as having constitutional or idiopathic aplastic anemia or myelodysplastic syndromes. Hoyeraal-Hreidarsson Syndrome is a severe form of X-linked DC with microcephaly, growth and mental retardation, spastic paresis, ataxia and immunodeficiency. In all forms, telomere protection or maintenance is defective.